For image display in an image display apparatus such as a PDP, that performs binary light emission, a so-called subfield method is used, where motion pictures with intermediate gradations are presented by superimposing chronologically a plurality of binary images each weighted.
In the subfield method, a single field is divided into a plurality of subfields, where each subfield weighted with luminance. The luminance weight for a subfield corresponds to the amount of light emission when the subfield is lighted. In other words, each subfield has a predetermined number of light-emission as its luminance weight, where the sum of the luminance weights of light-emitting subfield corresponds to a luminance gradation level to be displayed.
FIG. 10 illustrates a single field divided into eight subfields (SF1, SF2, . . . , and SF8). In FIG. 10, respective subfields have luminance weights 1, 2, 4, 8, 16, 32, 64, and 128. Each subfield has: setup period T1 for a preliminary discharge; address period T2 for an address discharge that sets to emitted or non-emitted for each pixel; and sustain period T3 during which pixels with emitted data being written by a discharge are made to emit light by generating a sustain discharge all at once. Here, light-emission of subfields occurs from SF1 through SF8 sequentially.
In the example shown in FIG. 10, light-emitting these subfields in various combinations represents gradations of 256 levels 0 through 255. For example, emitting SF1, SF2, and SF3 represents the gradation level 7 (1+2+4=7); SF1, SF3, and SF5, the gradation level 21 (1+4+16=21).
In this way, the subfield method represents multilevel gradations by dividing a single field into a plurality of subfields, and by selecting and light-emitting subfields from among a plurality of subfields to achieve a desired gradation.
In such a display apparatus that uses the subfield method for multilevel gradation display, it is known that false contour lines (hereinafter abbreviated as “dynamic false contours”) appear while displaying motion pictures. Next, a description is made for the dynamic false contours.
FIG. 11 illustrates how image pattern X horizontally moves on the screen of PDP 33. For example, the following situation is assumed. That is, one field is divided into subfields weighted as (1, 2, 4, 8, 16, 32, 64, and 128) and as shown in FIG. 11, image pattern X horizontally moves on the screen of PDP 33 b two pixels per one field. Image pattern X includes pixels P1 and P2, both with the gradation level “127,” and pixels P3 and P4, adjacent to pixels P1 and P2, both with the gradation level “128.” FIG. 12 is a view in which image pattern X is developed to subfields.
In FIG. 12, the lateral direction represents a horizontal direction on the screen of PDP 33, and the vertical direction represents a time direction. Further, the hatched areas show emitting subfields, and the non-hatched areas show non-emitting subfields.
In FIG. 12, when image pattern X remains stationary, the pixel-original gradation can be perceived because a viewer's sight line does not move (A-A′ in the figure). However, if image pattern X moves horizontally as shown in FIG. 11, a viewer's sight line follows image pattern X to move in directions B-B′ or C-C′ in FIG. 12. When the sight line moves in direction B-B′, the viewer sees SF1 through SF5 of pixel P4, SF6 and SF7 of pixel P3, and SF8 of pixel P2. In FIG. 12, these subfields, all non-emitting, end up in time-integrated, the gradation level 0 being viewed. Meanwhile, when the sight line moves in direction C-C′, the viewer sees SF1 through SF5 of pixel P1, SF6 and SF7 of pixel P2, and SF8 of pixel P3. In FIG. 12, these subfields, all light-emitting, end up in time-integrated, the gradation level 255 being viewed. In either case, where its gradation level largely differs from its original gradation level (127 or 128), the difference is perceived as false contour lines, which is deterioration in image quality. This phenomenon, dynamic false contours, occurs when pixels lie next to each other with such gradations that the pattern of emitting subfields largely changes on the contrary to its small change in gradation. In the example of the subfields weighted as described above, for adjacent pixels with luminance gradation levels 63 and 64, 191 and 192, and the like, dynamic false contours are notably observed also.
A description is made for a conventional method to suppress the dynamic false contours. First, convert the gradation level of an input image to a level at which dynamic false contours are unlikely occur, namely to a “predetermined gradation level” where the change in pattern of emitting subfields is small. Next, diffuse the difference between the converted gradation level and its pre-converted one, to the surrounding pixels. This interpolates the difference of the gradation levels caused by the conversion. If the difference is great between the gradation level of an input image and the “predetermined gradation level,” convert to an “intermediate gradation level,” which is between the gradation level of an input image and the “predetermined gradation level.” Next, add the difference of gradation levels between the intermediate gradation level” and the “predetermined gradation level,” to the “intermediate gradation level,” or subtract from the “intermediate gradation level.” Repeat the addition and subtraction alternately by dot, by line, and by field to present averagely “intermediate gradation levels.” In this way, in addition to a “predetermined gradation level” at which dynamic false contours are unlikely to occur, using an “intermediate gradation level” suppresses dynamic false contours while preventing the number of gradation levels to be reduced.
However, the above-mentioned conventional method has the following problems. That is, if gradations have some gradient, and also a part where such a condition applies over such a plurality of pixels that they are well perceived visually, for example an unfocused part of the image, moves at a speed visually traceable, very large dynamic false contours are observed. Further, in order to suppress the dynamic false contours near a gradation level at which they occur, the number of gradation levels must be reduced, disabling the number to be sufficiently secured.